test: strip all trailing zeroes from the protocol strings

This code worked because the only string we compared against was
"ei_connection_setup" which is 19 actual bytes.

test/eiproto.py.tmpl

@@ -219,7 +219,7 @@ class Interface:
                 if not s:
                     s = None  # zero-length string is None
                 else:
-                    s = s.decode("utf8")[:-1]  # strip trailing zero
+                    s = s.decode("utf8").rstrip("\x00")  # strip trailing zeroes
                 results.append(s)
             else:
                 results.append(values.pop(0))